Animal models suggest that dissipation of body heat is an important physiological process that may affect energy expenditure, and thus may possibly modulate body weight. It is unknown whether obesity and heat retention have a strong relationship in humans, and if so, whether deficient dissipation of heat contributes to human obesity or obesity induces deficits in heat dissipation. We propose to study obese (Body Mass Index greater than or equal to 30 kg/meter-squared) and normal weight (BMI less than or equal to 25 kg/meter-squared) adults to determine possible obesity-related differences in 1) regional body temperature heterogeneity (i.e., presence of localized areas of heat retention), 2) the extent to which locally retained heat may be co-localized with deep fat depots, 3) the effectiveness of the hands as dissipaters of heat, and 4) the ability of approaches that alter heat dissipation to modify deep-body temperature heterogeneity. It is hoped that the results of this study will provide preliminary evidence for future studies that attempt to facilitate weight loss in obese subjects through effective, guided applications of heat management. From a thermodynamic prospective, obesity is the result of an energy imbalance over time and can develop whenever energy intake exceeds energy expenditure by even a small margin. Energy exists as organic molecules and heat in living organisms. In adult mammals, body weight and fat content remain unchanged if energy intake is strictly equivalent to energy expenditure. In other words, regulation of body weight in adults requires that energy from food not required for physical activity, normal metabolism, or replacement of body tissues be entirely dissipated as heat. It is also accepted that individuals differ in their efficiency in using food, i.e. their ability to metabolize foods and store fat or burn nutrients releasing heat. Mammals try to limit tissue temperatures to approximately 40 degrees C by controlling heat production and dissipation. When local heat dissipation rates are not sufficient to maintain temperature at physiological levels, the result could be slowing of local metabolism and increased storage of fat in affected areas. A corollary would be that enhanced heat dissipation within the body could facilitate burning, rather than storage, of fat. We hypothesize that enhanced heat dissipation might increase the rate at which nutrients are able to be burned as fuel. Over the past year the concept of the study and protocol were developed and several specific methods to study human thermogenesis and human reaction to local cold stress adopted. Two of them are described below. Both concept and protocol have passed internal review of the Obesity unit, NICHD. 1. Recovery-enhanced infrared imaging of the fingers. Several subjects' responses to a mild cold challenge of hands in water were assessed using IR. Each subject was comfortably positioned in a temperature- and humidity-controlled room (typically 23 degrees C plus or minus 1 degree C and 60% humidity) for 20 min before taking measurements. Hands, protected from moisture by thin, disposable, gloves were then immersed in water at 15 degrees C for 2 min. Water temperature was controlled by calibrated cooling units (model ProThermo, ThermoTek, TX, USA). The wet gloves then were removed and the hands placed in the same position as before the cold stress. The IR imagery consisted of recording five IR images of the hands at rest, followed by recording of approximately 40 images during the rewarming period. The rewarming images were taken beginning immediately after the cold stress and then every 30 seconds. Each recording session lasted approximately 25 min, excluding the time needed for the cold stress. Rewarming curves were reconstructed from images for each of the five fingers of both hands; and the temperature integral Q was calculated over the 20 min following the cold stress. To evaluate the reproducibility of the IR findings the measurements were repeated on seven normal weight subjects on different days. The two series of data will be statistically compared. IR data are in the process of being analyzed by a one-way ANOVA test, and the statistical significance level will be fixed at p =0.01. Preliminary analysis shows that different subjects exhibit different reactivity times to cold stress, reflecting the tonus of the blood vessels of their fingers. 2. Microwave thermometry. Microwave thermometry is based on the physical fact that electromagnetic radiofrequency waves of 1.1-1.2 GHz (wavelength of 1-20 cm) are emitted by all living tissues. Biological tissue is relatively transparent to this energy. Radio waves with an intensity of approximately 3 x 10 exp-13 can typically be registered emanating from the human body. The method allows measurement of the internal temperature of human tissue at a depth of up to 5 cm, (from 32 - 38 degrees C) with an accuracy of plus or minus 0.3 - 0.4 degrees C. The device (model RTM-01-RES, EM Diagnostics, Arkansas, USA) is non-invasive and does not deposit energy into the patient. Rather, it is sensitive to energy-emission from the patient by using both microwave (RF) and spot infrared (IR) sensors. We are at an early stage in evaluating this novel technology for measurement of core body temperature gradients. Whole-room fabric shielding designed to minimize stray electromagnetic radiation was evaluated. Future experimentation will include: (a) Multiple measurement sessions for each individual subject to determine reproducibility. A "point to point" map of the temperature gradients will be collected (the device converts the "derived" energy into a temperature measurement). Due to the antenna's sensitivity and directionality the device has the ability to measure temperature gradients much deeper (5-6 cm) in tissue than can be measured using conventional contact or IR sensors. (b) Multipoint measurements to establish a deep baseline temperature and to characterize perfusion of fat depots during functional tests (mild cold challenge of fingers and oral glucose administration). (c) Deep temperature microwave measurements and measurement of the surface (skin) temperature at the same site by an IR sensor possessing an accuracy of plus or minus 0.2 degrees C (point to point mapping) and by an IR camera. Both the RF (internal) and IR (surface) information will allow derivation of individual temperature profiles of the regions of interest. This methodology will allow monitoring of intensity for both deep and surface heat sources, thermal gradients between internal and surface heat sources, vectors of the gradients, rates of heat dissipation and correlation of temperature measurements relative to the anatomical positions where temperature data are taken.